1. Field of the Invention
The present invention relates to a phase correction apparatus, a DVOR (Doppler VHF omnidirectional radio range) apparatus, and a phase correction method for radio navigation.
2. Description of the Related Art
An example of a DVOR apparatus for providing aircraft with directional information is disclosed in U.S. Pat. No. 4,484,196. The DVOR apparatus of the related art arranges, as shown in FIG. 5, a central carrier antenna A and forty-eight sideband antennas B1 to B48 along a circle having a diameter of about 13.5 m (a radius R of about 6.7 m in FIG. 5) around the carrier antenna A.
The carrier antenna A radiates a reference phase signal in all directions. The reference phase signal is an AM wave formed by amplitude-modulating at 30 Hz a carrier of 108 to 118 MHz. The sideband antennas B1 to B48 arranged along the circle are sequentially activated at regular intervals of, for example, 30 times a second, so that the sideband antennas B1 to B48 successively emit subcarriers whose frequency is higher than that of the carrier by, for example, 9960 Hz. A distance between the sideband antenna emitting a subcarrier and an optional spatial point periodically changes with time, and therefore, the subcarriers received at the optional spatial point periodically change the frequency thereof due to Doppler effect, to form an FM wave of 30 Hz. The phase of this FM wave is dependent on an orientation with respect to a DVOR station where the DVOR apparatus is present. Namely, the sideband antennas B1 to B48 radiate the FM wave superposed by a variable phase signal.
The reference phase signal and variable phase signal are adjusted so that their phases agree with each other on magnetic north, i.e., at zero degrees. An aircraft receives these two signals, detects a phase difference between the AM wave and the FM wave both modulated at the same frequency of 30 Hz, and finds a present orientation of the aircraft.
FIG. 1 shows a sideband transmission system of the DVOR apparatus according to the related art. In FIG. 1, a sideband transmitter 101 generates a half-sine wave signal, supplies the signal to a distributor 3, and controls the switching of the distributor 3 so that the half-sine wave signal is successively supplied to odd-numbered sideband antennas B1, B3, . . . , and B47. A sideband transmitter 102 generates a half-cosine wave signal, supplies the signal to the distributor 3, and controls the switching of the distributor 3 so that the half-cosine wave signal is successively supplied to even-numbered sideband antennas B2, B4, . . . , and B48. The sideband antennas B1 to B48 are connected through antenna cables C1 to C48, respectively, to the distributor 3.
FIG. 2 is a timing chart showing the timing of switching the sideband antennas. As indicated with waveforms (a) to (e) in FIG. 2, each of the 48 sideband antennas receives a signal for 1/720 seconds, i.e., the signals are supplied 30 times a second.
Waveforms (e.g. (a), (c), (e)) provided by the odd-numbered sideband antennas must be continuous between two adjacent ones. Also, waveforms (e.g. (b), (d)) provided by the even-numbered sideband antennas must be continuous between two adjacent ones. If waveforms are discontinuous between adjacent odd- or even-numbered antennas, an aircraft is unable to correctly detect a phase difference between the AM wave from the carrier antenna A and the FM wave from the sideband antennas B1 to B48, and therefore, is unable to find a correct orientation.
To avoid the problem, the DVOR apparatus according to the related art precisely equalizes the lengths of the antenna cables C1 to C48 with one another, to align the phases of radio waves provided by the sideband antennas B1 to B48.
However, to align the phases of radio waves provided by the sideband antennas, the 48 antenna cables C1 to C48 of the DVOR apparatus must precisely be processed into identical electrical lengths. This process needs a long time and skill. Even with the identical electrical lengths, the related art is still vulnerable to phase shifts that may occur due to the aging of the antenna cables after installing the DVOR apparatus at a site.